The demodulation of group 3 facsimile signals and transmission over digital channels in the baseband offers substantial transmission gain and transmission quality enhancement advantages.
Improvements in transmission quality with group 3 facsimile signals transmitted over digital channels in the baseband result because of the shorter analog links requiring equalization. This assumes that the FIUs are placed in the vicinity of the user facsimile machines, and that a relatively error free digital link is available.
Transmission gain benefits are realized when group 3 facsimile signals are transmitted in the baseband since the user information need only occupy 9.6 kbit/s worth of transmission capacity, as opposed to, for example, the 40 kbit/s worth of transmission capacity that would be necessary if ADPCM links (according to the 1988 Blue Book, CCITT Recommendation G.723) were used. When 64 kbit/s PCM links according to CCITT Recommendation G.711 are employed, the transmission gains are even more substantial. However, in addition to user originated data, line state transitions also need to be transmitted over the digital links. This requires the allocation of substantial additional transmission capacity (e.g., 10% of the capacity allocated for the transmission of user-data information) if an out-of-band approach is pursued.
For example, U.S. application Ser. No. 07/720,990 filed on Jun. 26, 1991 by the present inventors describes a communication system in which demodulated facsimile messages are transmitted over a digital transmission channel in a digital frame comprising 35 bits and appearing every 3.33 ms. Of these 35 bits, 32 bits are available for carrying the actual message, with the remaining 3 bits being set aside to carry information concerning the type of traffic (called "line state information") in the 32-bit message. These 3 bits are referred to as line control bits. With this scheme, the digital frame is transmitted at a rate of 10.5 kbits/sec, whereas the transmission rate of the actual message (i.e., 32 bits) is 9.6 kbit/s. Accordingly, this 35-bit digital frame is an example of an out-of-band approach which requires additional transmission capacity.
U.S. Pat. No. 4,771,335 to Obara discloses a communication system which includes two digital terminal stations, a digital communication line for connecting the two digital terminal stations, and two analog communication lines for connecting two facsimile devices to the two digital terminal stations, respectively. Each of the digital terminal stations includes a High-Level Data link Control (HDLC) unit which serves to assemble data into a digital packet and to disassemble digital packets received from the other digital terminal station. The method of assemblage and disassembly is based on the X.25 standard, established by the CCITT. According to this standard, a series of data is divided into several blocks each composed of a designated number of words, e.g., 256 octets. Control information, called a header, which is necessary to transfer the data is added to each block of data. The control information includes the address information and frame error check sequence information. According to the Obara system, the control information also includes an identifier which identifies the type of data. Thus, similar to the U.S. Application discussed above, the Obara system sets aside a portion of the transmitted data to carry information concerning the type of traffic being transmitted.
The following are further examples of data communication systems:
U.S. Pat. No. 4,897,835 discloses a paging communication system for transmitting data packets including system control packets and message packets to several receivers each having an address.
U.S. Pat. Nos. 4,817,087, 4,916,691 and 4,918,718 disclose packet assemble and disassemble circuitry.
U.S. Pat. No. 4,809,297 discloses an interface device for connection between a facsimile machine and a radio transceiver.
U.S. Pat. No. 4,894,843 discloses a data communication system including a control circuit which generates procedure signals in accordance with CCITT recommendations.
U.S. Pat. No. 4,855,995 discloses a data communication system including a device for generating a frame having several channels, and at least one of the channels includes a region for data information and a region for indicating the validity of the data region.